1. Field of the Invention
The present invention relates to an imaging device for photoelectrically reading visual information such as character information, image information, or the like.
2. Description of the Related Art
There are known systems for either reproducing radiation image information of a subject such as a human body on a photographic photosensitive medium or the like, or outputting the radiation image information as a visible image on a CRT or the like, using a stimulable phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to applied stimulating rays such as visible light, emits photo-stimulated light in proportion to the stored energy of the radiation.
Specifically, radiation image information of a subject such as a human body is temporarily recorded on a stimulable phosphor sheet which has a stimulable phosphor layer. When a stimulating light beam such as a laser beam or the like is applied to the stimulable phosphor sheet, the stimulable phosphor sheet emits photo-stimulated light that is commensurate with the recorded radiation image information. The photo-stimulated light is photoelectrically read and converted into an image signal, which is processed to output (reproduce) a visible image based on the radiation image information on a recording medium such as a photographic photosensitive medium or a display unit such as a CRT. Various apparatus which record and output such radiation image information are used in the art.
The above apparatus incorporate a reading assembly which comprises a stimulating system for applying a stimulating light beam to a stimulable phosphor sheet to scan the stimulable phosphor sheet two-dimensionally, and a light collecting system for photoelectrically reading photo-stimulated light emitted from the stimulable phosphor sheet in response to the applied stimulating light beam. The light collecting system generally comprises photoelectric transducers such as CCDs (Charge-Coupled Devices), for example.
The photo-stimulated light emitted from the stimulable phosphor sheet spreads to a size which is more than twice the diameter of the stimulating light beam. The CCDs have a pixel width (ranging from several μm to several tens μm) which is too small to detect the photo-stimulated light sufficiently. When the stored radiation image information is read from the stimulable phosphor sheet, therefore, the reading efficiency tends to be low due to the small pixel width of the CCD, and the low reading efficiency is liable to cause a reduction in the quality of an image which will be produced from the read radiation image information. This is because the effective amount of light detected by the CCD governs the signal-to-noise ratio of the resulting image, and the stimulating light beam spreads in an auxiliary direction which is substantially perpendicular to the array of pixels of the CCD, greatly reducing its amount of light falling on the CCD.
It has been proposed, as shown in FIG. 9 of the accompanying drawings, to employ a conical multiple fiber assembly 4 disposed on an entrance surface 2a of a CCD 2 and having a larger-diameter end 4a and a smaller-diameter end 4b facing the entrance surface 2a. Light which has entered the larger-diameter end 4a of the conical multiple fiber assembly 4 leaves from the smaller-diameter end 4b thereof, and is focused on the entrance surface 2a. 
The conical multiple fiber assembly 4 has a slanted circumferential surface extending from the larger-diameter end 4a toward the smaller-diameter end 4b. Therefore, light travelling in the conical multiple fiber assembly 4 and applied at a small angle to the slanted circumferential surface is not totally reflected within the conical multiple fiber assembly 4, but tends to leak out of the conical multiple fiber assembly 4. Accordingly, the light propagating efficiency of the conical multiple fiber assembly 4 is relatively low with respect to spreading light. The conical multiple fiber assembly 4 includes absorbent glass interposed between individual optical fibers, lowering the ratio of the optical fibers packed in the conical multiple fiber assembly 4. This structure makes it difficult or impossible to increase the light propagating efficiency of the conical multiple fiber assembly 4.